The present invention relates to dental alloys and, particularly, to non-precious dental alloys for use with procelain veneers in the manufacture of dentures, dental bridges and the like.
Metal alloys for dental prosthesis were, in the past, based on gold and/or silver, particularly on gold. The high cost of gold, however, due to its natural scarcity and competing uses in the field of jewelry has unnecessarily exaggerated the cost of gold-based dental prosthetic devices. In addition, the low tensile strength of gold makes this metal not entirely satisfactory in many respects. Further, although gold accepts readily a porcelain coating, it frequently requires a separate bonding layer because its natural oxide layer is not adequate in all respect for bonding porcelain.
A non-precious alloy which has been believed suitable for dental work, is described in the U.S. Pat. No. 3,459,545 as an alloy for turbine blades in gas turbine engines. However, it contains 1.8-3% tungsten, 0.5-2% columbium, 1-3% tantalum, 3-4% titanium and, as such, besides being rather expensive, it possesses an unduly high Brinnell hardness value, which makes the preparation and the finishing of the dental work exceedingly harduous. The melting point of this alloy is also unn considerably higher than that of a gold alloy.
Other known compositions, primarily based on nickel, include berryllium as a critical alloying element. Such alloys as those described in U.S. Pat. No. 3,749,570 have been criticized recently in that problems are encountered both in the preparation of the alloy and in the preparation of prosthetic devices because of the rather toxic nature of beryllium. The prior art has shunned these alloys as hazardous. However, from "An evaluation of non-precious alloys for use with procelain veneers -- part II -- industrial safety and biocompatibility", a study made by Drs. Moffa, Guckes, Okawa and Lilly of the U.S. Public Health Service Hospital, San Francisco, Calif. (The Journal of Prosthetic Dentistry, St. Louis, vol. 30, no. 4, part I, pp. 432-441, October 1973) it results that, to date, there have been no documented cases of beryllium toxicity of dental origin and that any dangers of toxicity are limited to certain operations in the preparation of dental prosthetic devices. Several Be-containing alloys have been tested, such as those sold under the trade names Vera Bond, Wiron, Gemini II, etc. but no detectable amount of Be concentration was found even during grinding -- the most prone operation to toxic environment -- and polishing of dental prosthetic devices, provided that the obvious precautions of employing an adequate exhaust ventilation system were observed.
Other alloys known in the art contain no iron, because iron results generally in a blackening of the prosthetic device. An example of such alloy is offered by U.S. Pat. No. 3,896,547. However, it has been found that, when properly combined with other elements, iron will enhance the bonding of the alloy to the porcelain veneer, while the lack of iron will invariably result in a poor bonding.
It is, therefore, an object of the present invention to overcome the disadvantages of the alloys of the prior art by providing a non-precious alloy which is resistant to oxidation, does not form black oxides, has a good coefficient of thermal expansion which is compatible with porcelains, has a Brinnell hardness and a melting point comparable to gold-base alloys and is easy to grind.